Magnetic disc drives are used for magnetically storing information. In a magnetic disc drive, a magnetic disc rotates at high speed and a transducing head "flies" over a surface of the disc. This transducing head records information on the disc surface by impressing a magnetic field on the disc. Information is read back using the head by detecting magnetization of the disc surface. The transducing head is moved radially across the surface of the disc so that different data tracks can be read back.
Over the years, storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the transducing head must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the transducing head. This is known as a "crash" and can damage the transducing head and surface of the storage disc resulting in loss of data.
From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping. Thus, there has been a search for alternative bearing assemblies for use with high density magnetic storage discs.
One alternative bearing design which has been investigated is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. In addition to air, typical lubricants include oil or ferromagnetic fluids. Hydrodynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat runout.
However, hydrodynamic bearings themselves suffer from a number of disadvantages based on how they are designed. For example, a single plate cantilevered bearing has a low stiffness-to-power ratio and could be sensitive to external vibration, imbalances and shock.
A desirable solution to this problem would be to have the spindle motor attached to both the base and the top cover of the disc drive housing. This would increase overall drive performance. A motor attached at both ends is significantly stiffer than one held by only one end.
All of the known hydrodynamic motor designs provide no method for top cover attachment. The reason for this is that in order to have top cover attachment, the motor and specifically the bearing would need to be open on both ends. Opening a motor at both ends greatly increases the risk of oil leakage out of the hydrodynamic bearing. This leakage is caused by, among other things, small differences in net flow rate created by differing pumping pressures in the bearing. If all of the flows within the bearing are not carefully balanced, a net pressure rise toward one or both ends may force fluid out through the capillary seal. Moreover, due to manufacturing imperfections of the bearing, the gap in the journal may not be uniform along its length and this can create pressure imbalance in the bearing--and hence, cause leakage when both ends of the hydrodynamic bearing are open. The net flow due to pressure gradients in a bearing has to be balanced (by all the journals individually) for the fluid to stay inside the bearing. Any imbalances due to pumping by the grooves of the journals will force the fluid out of the capillary until the meniscus at one end moves to a new equilibrium position. This dynamic equilibrium position of the meniscus will depend among other things on the speed (N), journal radius (R), journal gap (H), and on the amount of taper that comes from tolerance on cylindricity.
To minimize the effects of pressure imbalances, the bearings need to be decoupled from each other as much as possible. One of the ways to do so is to vent the bearings to the atmosphere at their ends. One of the reasons for this kind of venting in the bearing is that the effect of any kind of pressure imbalances or flow imbalances can be localized. If the bearing is not adequately vented in this way, there are chances that it will leak from the bearing. Thus, a need exists for a new approach to the design of a hydrodynamic bearing based motor to optimize stiffness (both radial and axial) and damping, and hence its dynamic performances. It is also desirable to design a hydrodynamic bearing which is open at both ends and which does not leak and which functions properly.